1. Field of the Invention
The present invention generally relates to the art of microelectronic circuit fabrication, and more specifically to a method for clean laser cutting of metal lines on microelectronic circuit substrates using reactive gases.
2. Description of the Related Art
Laser cutting or "zapping" is an effective method of programming gate arrays for Application Specific Integrated Circuits (ASIC), and otherwise modifying interconnect metallization patterns on microelectronic integrated circuits. An introduction to laser cutting is presented in an article entitled "Technologies for economic production of ASICs", by Meir Janai, in Solid State Technology, March 1993, pp. 35-38.
Metallization patterns for ASICs and other configurable integrated circuits are formed with "fuse" sections that can be cut to program or configure the chip for a specific application by disconnecting redundant circuitry. ASICs generally include thousands of such fuse sections. Fuse technology is also used for applications such as SRAM development, Field programmable devices (FPGA's, etc.) and circuit encrypttion.
The prior art method of laser cutting or zapping is illustrated in FIGS. 1 to 3. In FIG. 1, a metal interconnect line 10 is formed of an electrically conductive metal on an integrated circuit substrate 12. Where the substrate 12 is silicon, the line 10 is typically formed of aluminum or an alloy thereof.
The line 10 is part of a programmable gate array or other microelectronic circuit structure, and interconnects two logic gates or other circuit elements (not shown) that are to be disconnected in order to configure or program the circuit. An encapsulation or passivation layer 14, preferably of silicon dioxide, is formed over the line 10 in a "tunnel" configuration.
The line 10 is cut by applying an energy beam 16 to the central section thereof as indicated by arrows. An appropriate source of energy is coherent light emitted by a Nd:YAG laser 18. The energy applied by the laser beam 16 vaporizes the central section of the aluminum line 10, thereby cutting the line 10 and providing the desired circuit disconnection.
As illustrated in FIG. 2, the applied energy vaporizes not only the aluminum material of the line 10, but also the overlying silicon dioxide layer 14 and a generally small thickness of the underlying substrate 12. The vaporized material is predominantly aluminum from the line 10, however, and forms a gaseous cloud 20 in and over a space 22 that is created by removal of the central section of the line 10.
After the line 10 is cut, the laser beam 16 is moved to another area of the circuit to cut another line. After this occurs, a portion of the cloud 20 solidifies to form a residue layer 24 in the space 22 as illustrated in FIG. 3.
In the prior art, laser cutting is performed in an ambient of clean air, which comprises oxygen and nitrogen. The residue layer 24 is formed by explosive abulation of the aluminum metal, and comprises splattered aluminum particles having partially oxidized surfaces. These particles are electrically conductive, and can bridge the gap between and electrically interconnect the adjacent ends of the cut line 10.
This defeats the purpose of the laser cutting, since the line that was cut remains conductive after the operation is completed. Although not specifically illustrated, the residue layer 24 can also spread out to undesirably interconnect adjacent lines that were previously not and should not be interconnected.